GABA action in retina is mediated by GABA-A, GABA-B and GABA-C receptors. Although GABA-A and GABA-C receptors share certain features, the GABA-C receptors have a more prolonged and sensitive response to GABA and display a pharmacological profile of agonists and antagonists that is distinct from the other GABA receptors. In addition, GABA-C receptors appear to be retina-specific, and are expressed by bipolars and horizontal cells. The molecular composition of GABA-C receptors is controversial. Previously, the applicant's laboratory has cloned two novel GABA receptor subunits, rho1 and rho2, from bovine retina and found them to have structural similarity to the GABA-A receptor. Interestingly, expression of rho1 and rho2 in Xenopus oocytes results in homo-oligomeric receptors with electrophysiological properties similar to GABA-C receptors. Since rho subunits are expressed in retinal cells that exhibit GABA-C properties, GABA-C could be made up of rho subunits. Bipolars that express GABA-C also contain GABA-A and glycine receptors. Therefore, GABA-C receptors could be a mixture of subunits from different classes of receptors. The goal of this proposal is to determine the role the rho subunits play in the formation of mammalian retinal GABA-C receptors. The applicant will use a molecular approach to examine which subunit combinations create receptors with properties matching those of GABA-C receptors identified in vivo. This information will guide a search for amino acid sequences that specify assembly of subunits into GABA-C receptors. Finally, the applicant will test for specific subunit combinations in vivo by immunoprecipitation of bovine retina.